This extension constantly breaks. It's especially buggy after Firefox updates, but there seems to be no real rhyme or reason. Sometimes restarting Firefox fixes it. Sometimes I have to delete and reinstall the extension. I've done the latter several times.
Brokenness can include not adding new passwords (great, so I generate a non-human-readable password and then the extension says it won't save it...) or the dropdown showing up as a completely blank white box.
Frequently, even the vault will break on Firefox so I have to switch browsers to access/save my passwords and to get access. LastPass doesn't play nicely with Firefox and I really wish it would. When it works, it's great. But it it's about 60/40 chances that it does.